Looking into the code of Webapp Creator, we’ll find the code to import an image to be used as icon. Tapping on the place holder will open the Content Hub that will let us choose from where to import the image (see the Webapp Creator source code)

contentType is passed in Main.qml as ContentType.Pictures. So, we will only see apps from which we only can import images. handler is passed in the same line as ContentHandler.Source. As we want to import an image from the app selected in the Content Hub.

So, with this example, uMatriks will be able to be the destination, source and share app for all kinds of ContentType.
What about the other hooks in the manifest.json? That is discussed in the next guide.